Generally, a power tool having a battery pack has the ability to display the remaining battery capacity. In ordinary situations, the displayed remaining battery capacity is obtained by directly measuring the voltage between the two ends of the battery. When the battery is discharging, the temperature of the battery increases and the inner resistance varies according to the temperature. Because of the variance, the measured voltage does not always accurately reflect the energy of the battery for the motor and results in an incorrect display of the remaining battery capacity.
Also, this kind of detecting and indicating may be performed before the battery pack is used to drive the motor to operate. After the motor starts operating, the battery capacity indicating device may be turned off. As a result, when the power tool starts operating, the user does not know the remaining battery capacity of the battery pack. Because the parameters of the battery pack are not monitored during use, if the battery pack is over-current or over-temperature, the power tool will not create an alert or stop operating, which could harm a user.